Siliceous offretite

ABSTRACT

Zeolite crystals having an offretite structure have smaller crystals and/or can be produced using a synthesis mixture of lower alkalinity than could otherwise be used, if a divalent metal cation, such as magnesium, barium or cobalt is included in the synthesis mixture.

This is a continuation of application Ser. No. 08/108,554, filed Sep.30, 1993, now abandoned, which is a continuation of Internationalapplication Ser. No. PCT/EP92/00370.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to zeolite crystals with an offretitestructure, which crystals contain a divalent metal cation and processesfor producing such zeolites and a process for using the zeolites in theseparation or conversion of hydrocarbons.

2. Description of Related Art

One of the zeolitic structures which occurs naturally is offretite.Naturally occurring offretite is rare, and it is known to produceoffretite synthetically. For example, U.S. Pat. No. 4,259,174 describesa synthetic offretite which is a crystalline aluminosilicate comprisingtetramethylammonium, sodium and potassium ions. U.S. Pat. No. 4,834,961and GB2119352 have similar disclosures. Offretite may then be ionexchanged to introduce a metal cation from group II or Group VIII of theperiodic table or manganese.

GB-A-2160188 describes the preparation of a synthetic offretitecontaining potassium ions and at least one ion selected from otheralkali or alkaline earth metals. The use of a mixture of potassium andsodium or a mixture of potassium, sodium and lithium ions is preferred.If the zeolite is to be used in acid catalysis then it may be exchangedwith cations such as magnesium. When the zeolite is to be used as acarrier of a hydrocarbon conversion catalyst, it is possible to carryout post-production alterations to the zeolite, e.g. to lower itspotassium content or to introduce other metal cations into the zeoliteusing known methods.

SUMMARY OF THE INVENTION

The present applicants have surprisingly found that the incorporation ofa divalent metal cation into the synthesis mixture which crystallises togive an offretite is advantageous. The inclusion of the divalent cationspeeds the rate of formation of the crystals and results in smallercrystals compared with a mixture which is identical but for the lack ofadded divalent cation and which is treated under the same conditions oftemperature, pressure and time.

Thus the present invention provides a zeolite with an offretite crystalstructure and which comprises a non-exchangeable amount of a divalentmetal cation.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows scanning electron micrographs of offretite crystalsproduced with and without the presence of divalent metal in thesynthesis mixture according to Example 1.

DETAILED DESCRIPTION OF THE INVENTION

Preferably the divalent metal cation is an alkaline earth metal fromgroup IIA of the periodic table, particularly magnesium, barium orcobalt.

The inclusion of a cation such as magnesium makes it possible to use asynthesis mixture which has a lower alkalinity, expressed as a molarratio of M'₂ O/SiO₂ (where M' is as defined hereinafter), than couldpreviously be used, and still obtain an offretite crystal structure. Asynthesis mixture of low alkalinity may be preferred to produce azeolite such as offretite since the product will be more siliceous andconsequently is more stable when used, e.g. as the basis for a catalystin a hydrocarbon conversion or separation process.

The offretite crystals of the present invention may be produced using anadaptation of standard procedures to produce zeolite crystals of anoffretite structure. Thus the present invention provides a process forproducing offretite crystals by crystallising a synthesis mixturecomprising sources of M'₂ O, Al₂ O₃, SiO₂, a tetramethylammonium (TMA)ion and a divalent metal cation M²⁺, where M' is an alkali metal whichis potassium or a mixture of potassium and sodium.

The source of alkali metal oxide M'₂ O, is conveniently introduced intothe synthesis mixture in the form of potassium hydroxide and optionallyalso sodium hydroxide e.g. as pellets.

The source of aluminum may be an alumina introduced into the reactionmixture as for example Al₂ O₃.3H₂ O, previously dissolved in alkali.However, it is also possible to introduce aluminum in the form of themetal which is dissolved in alkali.

The source of SiO₂ is generally silica, and this is convenientlyprovided in the form of a colloidal suspension of silica such as LudoxHS 40 available from E. I. Dupont De Nemours & Co. Colloidal silica solsare preferred since they result in less contaminating phases. However,other sources such as silicates may be used.

The tetramethylammonium ion is conveniently provided in the form of atetramethylammonium halide, such as the chloride.

The divalent metal cation may be a group Ib metal such as copper, agroup II metal, for example magnesium, calcium or barium, a group IVmetal such as lead or group VI, VII or VIII metal such as chromium,manganese, iron, cobalt or nickel. These metals may be introduced in theform of any convenient compound, for example as an oxide, hydroxide,nitrate or sulfate. Magnesium, barium and cobalt are particularlypreferred cations.

A typical synthesis process will comprise the formation of foursolutions:

an aluminate solution containing the source of alkali metal and alumina;

a colloidal silica solution;

a solution of tetramethylammonium halide;

a solution containing the source of divalent cation e.g. Mg(NO₃)₂.6H₂ O.

The solutions are then mixed in any order to produce the synthesismixture.

It is preferred that the synthesis mixture comprises the ingredients inthe following molar ratios:

    ______________________________________                                        M'.sub.2 O/SiO.sub.2 0.12 to 0.4                                              SiO.sub.2 /Al.sub.2 O.sub.3                                                                        7 to 13                                                  TMA/SiO.sub.2        0.04 to 0.12                                             H.sub.2 O/M'.sub.2 O 40 to 135.                                               ______________________________________                                    

Optionally these ratios are:

M'₂ O/SiO₂ 0.15 to 0.4, preferably 0.18 to 0.36, more preferably about0.2 to 0.28, e.g. about 0.23;

SiO₂ /Al₂ O₃ 8 to 12, preferably 9 to 11, e.g. about 10;

TMA/SiO₂ preferably 0.06 to 0.11, e.g. about 0.1; and

H₂ O/M'₂ O 40 to 100, preferably 60 to 80, e.g. about 70.

Suitable quantities of divalent depend on the particular cation used.The following quantities are given for guidance. If the cation ismagnesium or cobalt then as little as 15 ppm will suffice to produce theadvantageous effects on crystal size and formation rate. For example,amounts of 15 to 150 ppm, preferably 30 to 90 ppm, of magnesium orcobalt are suitable. These quantities are based on the weight of thesynthesis mixture.

On the other hand, if barium is used, then larger amounts of cation maybe necessary to produce the advantageous effects. For example, 250 to900 ppm, preferably 300 to 450 ppm of barium may be used.

The synthesis mixture is crystallised to produce the zeolite.Crystallisation is generally carried out in a sealed autoclave and thusat autogenous pressure. It is possible to employ higher pressuresresulting from higher temperature. Lower pressure (which corresponds tolower temperatures) will require longer crystallisation times.

Crystallisation time is related to the crystallisation temperature. Ingeneral, the higher the crystallisation temperature, the faster thecrystallisation. The crystallisation is usually carried out at atemperature of at least 120° C., preferably in the region of 120° to180° C., more preferably in the region of 150° C. At this temperaturethe crystallisation time may be from 50 to 250 hours, typically from 40to 80 hours.

Following this preparation the zeolite may be separated, washed anddried. The washing may be to a pH of more than 7, e.g. 9 to 10. Dryingmay be at a temperature of from at least 100° C. e.g. about 150° C. forabout 16 hours.

Scanning electron micrographs (SEM) of the products show that theyconsist of crystals which are approximately cylindrical or barrel-likein shape with a hexagonal cross-section. X-ray diffraction showed thatthe products are excellently crystalline and pure zeolite offretite.

The X-ray diffractograph showed no extra peaks at 20 values of 9.63,16.55 or 21.35, indicating that the crystals are free from stackingfaults.

FIG. 1 shows SEM's of offretite crystals produced with and without thepresence of Mg in the synthesis mixture.

The zeolites of the present invention are preferably aluminosilicatesand are described herein in terms of aluminosilicates, but thesubstitution of other elements is possible; for example, aluminium maybe substituted by gallium. However, the advantageous effect of the addeddivalent cation decreases with increasing substitution of gallium. It istherefore preferred that the zeolite is an aluminosilicate.

The zeolite may be used as a selective adsorbent in a hydrocarbonseparation or conversion process e.g. in catalytic de-waxing ofhydrocarbons or in a hydrocracking or a hydroisomerisation process. Theability to prepare crystals in accordance with the present inventionwhich are smaller than crystals produced without the addition of adivalent cation is beneficial since the smaller crystals will exhibit areduced mass transfer resistance. The operating temperature in theseparation process could therefore be reduced. Looked at from anotherpoint of view, the addition of a divalent cation to a synthesis mixtureenables one to use a more highly siliceous synthesis mixture (whichproduces a more stable product) and still obtain a small crystal size inthe product.

The zeolite may be composited with a binder and/or used in intimatecombination with one or more catalytically-active metals. Suitablebinders include e.g. silica, alumina, or a clay such as kaolin.

The catalytically-active metal may be for example tin, germanium,tungsten, vanadium, molybdenum, chromium, manganese or a group VIIImetal such as platinum, palladium, cobalt, nickel or a mixture of e.g.platinum and rhenium.

The metal may be combined with the zeolite by e.g. ion-exchange,impregnation or intimately mixing the metal(s) and the zeolite.

Accordingly, the present invention provides a process for separating orconverting hydrocarbons comprising the use as a catalyst of a zeolite asdescribed herein.

The present invention is illustrated by the following examples:

EXAMPLE 1

Preparation of small-crystal-size zeolite offretite by using traces ofMg²⁺ -species in the synthesis mixture.

Preparation of Synthesis Mixture

(All quantities of chemicals are given in grams)

    ______________________________________                                        Solution A: Aluminate Solution                                                KOH pellets (87.3% purity)                                                                        29.59                                                     Al(OH).sub.3 powder (98.6% purity)                                                                15.80                                                     H.sub.2 O           65.91                                                     Solution B: Colloidal Silica                                                  Ludox HS-40         150.23                                                    Solution C: Tetramethylammonium chloride                                      TMA-Cl              11.01                                                     H.sub.2 O           75.39                                                     Solution D: Magnesium Nitrate                                                 Mg(NO.sub.3).sub.2.6 H.sub.2 O                                                                    0.1277                                                    H.sub.2 O           49.29                                                     ______________________________________                                    

These 4 solutions were combined as follows:

Solution C was added to solution B and mixed to homogenize, solution Dwas added to the combined solutions C/B, and finally solution A wasadded to the combined solutions C/B/D. The whole was mixed for about 4minutes to ensure a homogeneous mixture.

The thus obtained mixture had a molar composition of:

2.30 K₂ O/Al₂ O₃ /9.96 SiO₂ /1.0 TMACl/161 H₂ O+30 wt ppm Mg based onthe weight of synthesis mixture.

Crystallization:

300.83 gram of the mixture was transferred into a carefully cleaned 300ml stainless steel autoclave and heated up to 150° C., the total heatingtime was 69 hours.

Washing and Recovery of the Product:

The product was washed with demineralized water to pH 10.5 andsubsequently dried at 125° C. during about 16 hours. The quantity ofproduct recovered was: 55.8 grams.

COMPARATIVE EXAMPLE A

Preparation of zeolite offretite without addition of Mg²⁺ -species inthe synthesis mixture.

A similar synthesis mixture was prepared using the same ingredients asin Example 1 with the exception of the Mg²⁺ -species. The molarcomposition of this mixture was:

2.30 K₂ O/Al₂ O₃ /9.96 SiO₂ /1.0 TMACl/162 H₂ O.

Crystallization:

299.85 grams of the mixture was crystallized under the same conditionsas in Example 1.

Washing and Recovery of the Product:

The product was washed to pH 10.4 and dried during about 16 hours at125° C. The quantity of recovered product was: 54.8 grams.

Characterization:

X-ray diffraction showed that both products were excellently crystallineand pure zeolite offretite. This indicates that, under the abovesynthesis conditions, the presence of Mg²⁺ -species in the crystallizingmagma does not trigger formation of by products such as zeolite-L,Philipssite etc.

SEM showed that the offretite crystals prepared with Mg²⁺ -species wereabout 4 times smaller than those obtained without Mg²⁺ species.Comparative 10000 times micrographs are shown in FIG. 1.

EXAMPLE 2

Synthesis details:

A similar synthesis was carried out using the same ingredients as inExample 1 and comparative example a. The synthesis mixture had thefollowing molar composition:

1.8 K₂ O/1.0 TMACl/Al₂ O₃ /10SiO₂ /160H₂ O+X ppm Mg²⁺ the mixture wascrystallised for 70 hours at 150° C. in a 300 ml stainless steelautoclave.

Details of the products are given in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                        PRODUCT                                                   SYNTHESIS                     SEM                                                     wt ppm Mg.sup.2+                                                                          XRD       Crystallite                                     K.sub.2 O/SiO.sub.2                                                                   in gel heating                                                                            % cryst.                                                                          contami-   length                                                                            dia                                    Ratio   (X)    time (hrs)                                                                         vs Ref.                                                                           nants habit                                                                              (microns)                                  __________________________________________________________________________    Ref.                                                                             0.18 nil    70   78  amorphous                                                                           hexagon-                                                                           4-10                                                                              2.5                                    Ex.b.                         al barrels                                      Ex. 2                                                                            0.18 30     70   94  none  hexagon-                                                                           1-3 0.5-1                                                                al barrels                                      __________________________________________________________________________

In the absence of magnesium, the product is 78% crystalline and containsamorphous contaminants. Using magnesium permits the production of a morecrystalline product (94% crystalline) without contamination indicatingthat the cation enhances the rate of formation of crystalline producti.e. the cation is a crystallisation promoter. Moreover, the crystalsare smaller.

EXAMPLE 3

Example 1 was repeated using 30 and 90 ppm of magnesium respectively.Thus the synthesis gel had the following molar compositions.

2.3 K₂ O/1.0 TMACl/Al₂ O₃ /10 SiO₂ /160H₂ O+30 or 90 ppm Mg²⁺.

The mixture was crystallised at 150° C. for 70 hours in a 300 mlstainless steel autoclave.

The mixture containing 30 ppm magnesium produced hexagonal barrel shapedcrystals of length 0.5 to 1.5 microns. The synthesis mixture containing90 ppm of magnesium produced hexagonal barrel shaped crystals of length0.5 to 1.2 microns. Thus an increase in the amount of magnesium used inthe synthesis gel does not further reduce significantly the size ofoffretite crystals.

EXAMPLE 4

The beneficial effect of adding magnesium can also be seen when thepotassium in the synthesis mixture is partly replaced by sodium.

A synthesis mixture was prepared using the ingredients of example 1, butin which potassium hydroxide was partly replaced by sodium hydroxide.Synthesis gels were produced having the following molar compositions:

1.15 Na₂ O/1.15 K₂ O/1.0 TMACl/Al₂ O₃ /10 SiO₂ /161 H₂ O (+30 ppm Mg²⁺).

The mixture was crystallised at 150° C. for 68 hours in a 300 mlstainless steel autoclave. Reference Examples were prepared in which noMg²⁺ was added to the synthesis mixture. Reference Example C used onlyKOH in preparing the mixture; Reference Example D used a mixture of KOHand NaOH. The results are given in Table 2.

                                      TABLE 2                                     __________________________________________________________________________               PRODUCT                 Prod                                                  XRD      SEM            yield                                      Synthesis  % Cryst                                                                           contami-   length                                                                            diameter                                                                           wt %                                       Mixture    Ref.                                                                              nants                                                                              habit (microns)                                                                              a)                                         __________________________________________________________________________    Ref C                                                                             K only 100 none hexagonal                                                                           3-5 2    16.4                                           no Mg.sup.2+    barrel                                                        added                                                                     Ref D                                                                             K/Na present                                                                         90  none sphere-like                                                                         --  -5   >16.0 (some product                            no Mg.sup.2+                   losses during                                  added                          washing).                                  Ex. 4                                                                             K/Na present                                                                         93  none sphere-like                                                                         --  1-3  16.4                                           30 ppm Mg.sup.2+                                                              added                                                                     __________________________________________________________________________     a) at calcined product/wt gel *100%                                      

EXAMPLES 5 to 8

The K₂ O/SiO₂ ratio in the synthesis gel was further reduced comparedwith Example 1.

Using the ingredients of example 1, synthesis gels were produced havingthe following composition:

without magnesium: (1.8-2.3)K₂ O/1.0 TMACl/Al₂ O₃ /10 SiO₂ /160 H₂ O.

with magnesium: (1.5-2.0)K₂ O/1.0 TMACl/Al₂ O₃ /(10-12.5) SiO₂ /160 H₂O+30 ppm Mg²⁺.

The synthesis mixtures were crystallised at 150° C. in a 300 mlstainless steel autoclave for the times shown in Table 3.

Reference Examples C, E and F were prepared in which magnesium was notadded to the synthesis mixtures, and in which the synthesis mixtures hadratios of K₂ O/SiO₂ which varied from 0.18 to 0.23. Examples 5 to 8 wereproduced from synthesis mixtures whose K₂ O/SiO₂ ratios varied from0.144 to 0.20 and to each of which mixture had been added 30 ppm ofMg²⁺.

Details of the ratio of ingredients and properties of the productsproduced are given in Table 3.

The SiO₂ /Al₂ O₃ ratio in the product was calculated. This is based onthe assumption that all the aluminium in the autoclave is in theproduct. Previous analyses have shown this to be a reasonableassumption. Knowing the quantities of other materials present andassuming that the weight of potassium in the product accounts for thetotal weight contributed by the potassium and TMA it is possible tocalculate the SiO₂ /Al₂ O₃ molar ratio. Because of the assumptions thecalculation is a conservative estimate of the ratio, and analysis showsthat the product is in fact, slightly more siliceous than the calculatedratios indicates.

In the absence of Mg²⁺ calculated molar SiO₂ /Al₂ O₃ ratios of 8.5 maybe achieved. When Mg²⁺ is added to the synthesis mixture this ratio maybe increased to as much as 9.9.

It can be seen from this that the inclusion of magnesium enables theproduction of offretite containing larger proportions of silica thanwould otherwise be possible for a particular crystal size.

                                      TABLE 3                                     __________________________________________________________________________                   PRODUCT                                                                                SEM           product                                                                           SiO.sub.2 /Al.sub.2 O.sub.3         SYNTHESIS      XRD      Crystallite   yield                                                                             Ratio                               K.sub.2 O/SiO.sub.2                                                                    Heating                                                                             % cryst                                                                           contami-  length                                                                            diameter                                                                           wt %                                                                              calculated                          Ratio    time (hrs)                                                                          vs ref.                                                                           nants                                                                              habit                                                                              (microns)                                                                              a)  from Product Yield                                                                     analyzed                   __________________________________________________________________________    Ref. C.                                                                           0.23 70    100 none hexagonal                                                                          3-5 2    16.4                                                                              >7.5     8.5                                                barrels                                               Ref. E                                                                            0.20 70    94  none hexagonal                                                                          3-8 2.4  >16.9                                                                             >8.0     N/A                                                barrels                                               Ref. F                                                                            0.18 70 (spot) (b)                                                                       78  amorph.                                                                            hexagonal                                                                          4-10                                                                              2.5  >17.5                                                                             >8.5     N/A                                 96    87  none barrels                                               __________________________________________________________________________                         PRODUCT                                                                                  SEM          product                                                                            SiO.sub.2 /Al.sub.2                                                           O.sub.3                     SYNTHESIS            XRD        Crystallite  yield                                                                              ratio calc                  K.sub.2 O/SiO.sub.2                                                                    SiO.sub.2 /Al.sub.2 O.sub.3                                                         Heating                                                                             % cryst                                                                           contami-  length                                                                             diameter                                                                           wt % from Prod.                  Ratio    Ratio time (hrs)                                                                          vs ref.                                                                           nants  habit                                                                            (microns) a)   Yield                       __________________________________________________________________________    Ex 5                                                                             0.20  10    70    101 none   hexa-                                                                            1-1.5                                                                              0.5-0.8                                                                            17.4 >8.2                        Ex 6                                                                             0.18  10    70    94  none   go-                                                                              1-2.5                                                                              0.5-1.0                                                                            18.0 >8.4                        Ex 7                                                                             0.15  10    120 (spot)                                                                          91  amorphous                                                                            nal                                                          230 (final)                                                                         101 none   bar-                                                                             2-5  1-2  18.5 >8.7                        Ex 8                                                                             0.144 12.5  120 (spot)                                                                          72  amorphous                                                                            rels                                                                             2-6  1-2  19.6 >9.9                                       230 (final)                                                                         98  none                                                 __________________________________________________________________________     a) wt calcined product/wt gel *100%                                           b) The autoclave was cooled down to freeze the system, a spot sample was      taken and the autoclave with then heated up, to continue the                  crystallisation process.                                                 

EXAMPLES 9 to 10

The use of barium cations was compared with the use of magnesium ions.Using the same ingredients as in example 1, synthesis mixtures wereproduced having the following molar compositions:

1.50 K₂ O/1.0 TMACl/Al₂ O₃ /10 SiO₂ /160 H₂ O+X ppm Mg²⁺ /Ba²⁺

The mixture was crystallised at 150° C. in a 300 ml stainless steelautoclave for the length of time indicated in Table 4.

The properties of the products are also indicated in Table 4. Theresults for Example 7, using a smaller amount of Mg²⁺ then Example 9 areincluded in Table 4 for comparison.

                                      TABLE 4                                     __________________________________________________________________________                    PRODUCT - OFFRETITF                                           SYNTHESIS                 SEM       Product                                              time into                                                                          XRD       Crystallite                                                                             Yield                                                                             SiO.sub.2 /Al.sub.2 O.sub.3           divalent cation                                                                          heating                                                                            % cryst                                                                           contami- length                                                                            dia                                                                              wt %                                                                              Ratio                                 type  conc. ppm                                                                          hrs  vs ref                                                                            nants habit                                                                            (microns)                                                                            a)  (calc)                                __________________________________________________________________________    Ex 7                                                                             Mg.sup.2+                                                                        30   120 (spot)                                                                         91  amorphous                                                                              2-5 1-2                                                                              18.5                                                                              >8.7                                             230 (final)                                                                        101 none                                                      Ex 9                                                                             Mg.sup.2+                                                                        62   120 (spot)                                                                         97  trace of 2-5 1-2                                                              amorphous                                                 Ex 10                                                                            Ba.sup.2+                                                                        355  93   96  none     1-2 0.5-1                                                                            18.4                                                                              >8.6                                  __________________________________________________________________________     a) wt calcined product/wt gel *100%.                                     

EXAMPLE 11

The addition of cobalt is shown to have a beneficial effect. Synthesismixtures were prepared using the same ingredients and methods as inExample 1, but cobalt nitrate was substituted for the magnesium source.The synthesis mixture of Example 11 had the following molar composition:

2.85 K₂ O/Al₂ O₃ /TMACl/10 SiO₂ /160 H₂ O+30 ppm Co²⁺.

The amount of Co²⁺ is based on the total weight of the synthesismixture. A comparative synthesis mixture (Reference Example G) wasprepared in the same way and having the same molar composition butwithout the added cobalt.

The synthesis mixtures were each placed in a 300 ml stainless steelautoclave, heated to 150° C. and maintained at that temperature for 70hours.

The products were each washed to a pH of 10.4 and dried.

X-ray diffraction showed that both products were offretite withexcellent crystallinity and had no amorphous contaminants. The crystalsizes of the products, assessed using SEM, are given in Table 5.

    ______________________________________                                                    xtal Length                                                                           ave. Length                                                           (μm) (μm)                                                   ______________________________________                                        Example 11    0.3-0.9   0.6                                                   Ref. Ex. G    1.5-4     3                                                     ______________________________________                                    

The crystals of Example 11 had a significantly smaller length and werecongruently reduced in size compared with the crystals of ReferenceExample G.

EXAMPLE 12

The effect of reduction of the K₂ O/SiO₂ ratio in the synthesis gel wasfurther examined in a synthesis gel using barium cations.

The same ingredients as in Example 1 were used, save for the use ofhydrated barium hydroxide, Ba(OH)₂.8H₂ O in place of magnesium nitrate,and by the same procedure a synthesis gel was produced having thefollowing composition:

1.3K₂ O/1.0TMACl/Al₂ O₃ /10SiO₂ /160H₂ O+714 wt ppm Ba²⁺. The synthesisgel was crystallised generally as described in Example 1 but held at150° C. for 10 hours. The product was washed to pH 10.1 and dried at150° C. for 10 hours.

A comparative synthesis gel was prepared in the same way but in whichthe K₂ O/SiO₂ ratio was further reduced to 0.11. Crystallisation wascarried out over an extended period and samples taken at 119 and 283hours when crystallisation was stopped.

X-ray diffraction showed that Example 12 gave offretite of excellentcrystallinity and purity. SEM showed the offretite crystals to bebarrel-shaped with a length of about 1.0 micron and a diameter of about0.6 micron.

Reference Example H gave only amorphous material after 119 hours heatingat 150° C. After a total of 283 hours crystallisation X-ray diffractionshowed only traces of offretite, the product being mainly zeolite W.

I claim:
 1. A process for producing a zeolite with an offretite crystalstructure and which contains a non-exchangeable amount of divalent metalcation, which process comprises crystallizing a synthesis mixturecomprising sources of M'₂ O, where M' is an alkali metal which ispotassium or a mixture of potassium and sodium, Al₂ O₃, SiO₂, atetramethylammonium ion and a divalent metal cation M²⁺, and comprisingthe following molar ratios:

    ______________________________________                                        M'.sub.2 O/SiO.sub.2 0.12 to 0.4                                              SiO.sub.2 /Al.sub.2 O.sub.3                                                                        7 to 13                                                  TMA/SiO.sub.2        0.04 to 0.12                                             H.sub.2 O/M'.sub.2 O 40 to 135,                                               ______________________________________                                    

wherein TMA represents said tetramethylammonium ion, said synthesismixture containing said divalent metal cation at a level sufficient topromote the formation of smaller zeolite crystals than an otherwiseidentical synthesis mixture which does not contain said added divalentmetal cation, said level not exceeding 900 ppm, when the divalent cationis barium.
 2. A process as claimed in claim 1 in which M is magnesium,and the synthesis mixture contains 15 to 150 ppm of Mg²⁺.
 3. A processas claimed in claim 1 in which M is barium, and the synthesis mixturecontains 250 to 900 ppm of Ba²⁺.
 4. A process as claimed in claim 1 inwhich M is cobalt, and the synthesis mixture contains 15 to 150 ppm ofCo²⁺.
 5. A process as claim in claim 1 in which the crystallisation iscarried out at 120° to 180° C. for 50 to 250 hours.
 6. The process ofclaim 1 wherein said divalent metal cation is selected from the groupconsisting of copper, magnesium, calcium, barium, lead, chromium,manganese, iron, cobalt and nickel.
 7. A synthetic zeolite with anoffretite crystal structure which contains a non-exchangeable amount ofa divalent metal cation, said zeolite crystallized from a synthesismixture comprising sources of M'₂ O where M' is an alkali metal which ispotassium or a mixture of potassium and sodium, and also containing saiddivalent metal cation at a level sufficient to promote the formation ofsmaller zeolite crystals than an otherwise identical synthesis mixturewhich does not contain said added divalent metal cation, said level notexceeding 900 ppm, when the divalent cation is barium.
 8. The syntheticzeolite of claim 7 wherein said divalent metal cation is selected fromthe group consisting of copper, magnesium, calcium, barium, lead,chromium, manganese, iron, cobalt and nickel.
 9. The synthetic zeoliteof claim 8 which is crystallized from a synthesis mixture containingmagnesium or cobalt cations at a maximum level of 150 ppm.
 10. Thesynthetic zeolite of claim 9 which is crystallized from a synthesismixture containing magnesium or cobalt cations at a level of from about30-90 ppm.
 11. The synthetic zeolite of claim 9 wherein said cation ismagnesium.
 12. The synthetic zeolite of claim 9 wherein said cation iscobalt.
 13. The synthetic zeolite of claim 7 which is crystallized froma synthesis mixture containing SiO₂ and Al₂ O₃ at a respective molarratio of 7 to 13:1.
 14. The synthetic zeolite of claim 8 which iscrystallized from a synthesis mixture containing barium cation.
 15. Thesynthetic zeolite of claim 14 which is crystallized from a synthesismixture containing 250 to 900 ppm barium.
 16. The synthetic zeolite ofclaim 15 wherein said synthesis mixture contains 300 to 450 ppm barium.17. The synthetic zeolite of claim 14 which is crystallized from asynthesis mixture containing SiO₂ and Al₂ O₃ at a respective molar ratioof 7 to 13:1.
 18. A catalyst comprising a zeolite as claimed in claim 7and a catalytically active metal.